Domestication of plants for human consumption has resulted in altering most crops and crop-related plant's ability to uptake metals from the soil medium. The ultimate aim of many decades of plant breeding practices has been to reduce the accumulation of unfavourable substances in plant parts that are destined for human or animal consumption. However, a few wild and native plant species have, to date, retained their ability to uptake undesirable toxic substances from the growth medium on which they establish. Some of these plant species often accumulate very high concentrations of metal ions in their foliage. These plants are commonly classified as "hyperaccumulators". Hyperaccumulators are plants which can accumulate toxic metal ions, such as nickel, copper, cobalt and lead at very high concentrations in shoot or root tissues (&gt;0.1% of the dry matter). These plants are normally found growing in soils containing unusually high concentrations of these metals in special geological formations, for example, the presence of zinc accumulating Thlaspi plants growing in zinc-rich soils near the Germany and Belgium border. To date, only a handful of hyperaccumulating plant species have been identified for their potential to uptake different metal species (see Table A.).
The term phytoremediation refers to the effective utilization of such metal-hyperaccumulating plant species which have the ability to uptake, bind, and detoxify environmental contaminants, such as metal ions and organics, through plant-mediated biological, biochemical and physical means. The current focus of researchers is to identify and select better plant species for phytoremediation, species that can be classified as hyperaccumulators and that also possess a large biomass into which the plants can accumulate and sequester large quantities of toxic metal ions. The identified plants must be hardy and suitable for the temperate North American environment. As stated by Brown et al. (1995), the hyperaccumulation mechanism involves the translocation of the metals from soil to shoot tissues in excess of 100 mg/kg for Cadmium, 1,000 mg/kg for Nickel and 10,000 mg/kg for Zinc, Copper and Cobalt hyperaccumulators are defined as plants capable of accumulating more than 0.1% (1,000 mg/kg) of these metals in their dried tissue (Baker et al., 1988).
TABLE A Metal concentrations in the known hyperaccumulator species [concentration in harvestable material from plants growing in contaminated soils (on dry weight basis)] Concentration Metal Plant Species [mg/kg in shoots] Cd Thlaspi caerulescens 1,800 Cu Ipomea alpina 12,300 Co Haumaniastrum robertii 10,200 Pb T. rotundzfoliium 8,200 Mn Macademia neurophylla 51,800 Ni Psychotria douarrei 47,500 Sebertia acuminata (25% by wt of dried sap) Zn T. caerulescens 51,600
The major limitations of utilizing these hyperaccumulating plant species for phytoremediation are:
a) Plants such as Thlaspi and Haumaniastrum are very small, with a very low plant biomass. Although these plants can uptake metals &gt;1% of their dry weight ("DW"), their low biomass limits their ability to uptake large amounts of metal ions. For example, shoots of T. rotundifolium can accumulate up to 8200 mg/kg DW of Pb but these plants can only produce 5 to 50 mg of plant dry material during a 5 month growing period. Therefore, these plants would have to be grown over several growth cycles and seasons in order to achieve complete remediation of a site. PA0 b) Plants such as Thlaspi and Haumaniastrum are very small in stature, and therefore are not amenable for harvesting using conventional farm machinery. PA0 c) Plants such as Thlaspi and Haumaniastrum have a very slow growth habit (Thlaspi rotundifolium has a 5 month growth period). A long growth cycle would result in longer remediation periods. PA0 d) Tree species such as Sebertia acuminata have a longer growth period but due to their tropical origin, they might not be able to over-winter in temperate environments, and hence may not be useful for phytoremediation purposes in North America. PA0 e) All the hyperaccumulator plant species mentioned above have specific target metal species, which they are capable of accumulating in very large amounts in their plant parts. However, most of the contaminated soil sites have a mixture of metal contaminants. In the presence of such complex metal contaminants, it is very unlikely that these known hyperaccumulators will be able to survive and uptake large levels of the different metal ions. For example, petroleum industries land-farming sites in Sarnia, Ontario, Canada, have a mixture of about 15 different metal ions and organic contaminants in varying concentrations, depending on the location. PA0 a) Most of the currently identified plants are wild relatives of the cultivated crop species Brassica napus (canola). Due to the potential for cross-pollination between the wild-relatives and crop species, public acceptance of these plants for phytoremediation is questionable. This problem holds credence considering the potential for evolution of new weed-like species, which might interfere with current agricultural systems. PA0 b) These plants set seeds readily and might assume weed-like characteristics after repeated growth in contaminated sites. PA0 c) These plants have relatively larger biomass than Thlaspi sp. However, they still do not compare well with plants with denser foliage (larger biomass). PA0 a) The plants (belonging to the genus Pelargonium, especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) ability to survive on soils contaminated with one or more metal ions. Most known hyperaccumulators have limited potential in phytoremediation, as they are suitable for remediating only specific (individual) metals. PA0 b) The plants' (belonging to the genus Pelargonium, especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) ability to uptake, translocate and accumulate a wide array of metal ions, such as cadmium, lead, zinc, copper, nickel in the shoot biomass. Most known hyperaccumulators can uptake only one specific metal ion, and therefore are limited in their applicability to remediate soils with complex metal ion mixtures. PA0 c) The plants (belonging to the genus Pelargonium, especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) possess a very dense foliage (consisting mostly of leaves) for sequestering high levels of metal ions in the above ground parts. The shoot biomass is exceptionally higher than any known hyperaccumulator e.g., Thlaspi sp., and plants belonging to Brassicaceae family. PA0 d) The plants (belonging to the genus Pelargonium, especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) have a faster and robust growth habit. Most known hyperaccumulators (e.g., Thlaspi sp.) have a very slow growth habit. Pelargonium sp. `Frensham` can attain a biomass of greater than 4 kg within 5-6 month growth period. PA0 e) The plants (belonging to the genus Pelargonium, especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) have an efficient and prolific root system that can efficiently absorb metal ions from the soil or ground water. Pelargonium sp. has a prolific root system, which can grow up to 3-4 feet within a 5-6 month growth period. PA0 f) The plants (belonging to the genus Pelargonium, especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) have the ability to grow in a wide variety of soils with relatively low requirements for water, nutrients and other conditions required to sustain growth and metabolism (they are also viable in different types of soil, soil factors, and in adverse environments unlike many other known hyperaccumulators). PA0 g) The plants (especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak`) have the ability to moderately retain senescing and dead leaves (without withering) thereby reducing recycling of metal ions to the contaminated soil, (a very distinct characteristic of certain scented geraniums). PA0 h) The use of harvested plants (especially Pelargonium sp. `Frensham`, Pelargonium sp. `Citrosa` and Pelargonium sp. `Beauty Oak` shoot biomass) for extraction of essential aromatic oils such as, citronellol, geraniol, iso-methane, geranyl formate etc. No other hyperaccumulator has been shown to have economic return from plants used for phytoremediation.
Among the hyperaccumulating plant species currently being considered for phytoremediation and which have been characterized in greenhouse and field conditions, the most promising ones are Thlaspi caerulescens and plants belonging to the Brassicaceae family.
The limitation of using plants belonging to the Brassicaceae family are:
The ability of plants to extract metal ions from soils and accumulate or sequester those metals in their tissues can be tremendously improved by adjusting the pH of the soil and also by the addition of synthetic chelators to the growing media. These two elements increase the release (desorption) of metal ions from soil particles, thereby increasing the availability of those ions to the plant roots, resulting in increased rate of uptake. The limitations of using metal chelators are:
The addition of chelating agents to metal contaminated soils could bring in new problems regarding health, safety and environmental concerns. Addition of large amounts of chelates will result in rapid solubilization of different metal ions. Some of the metals released will be beneficial to plants and microflora of the soil. However, this will also increase the soil solution concentration of undesirable ions. Moreover, there is a larger risk of releasing large amounts of the solubilized toxic metals in the underground water systems. The use of chelates would increase the bioavailability and uptake of these toxic metals by the natural flora and fauna of the soil, thereby accelerating the spread of these metals in the ecosystems and in the different food chains.